<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<div id="div1">

</div>
<script>
  var div1 = document.getElementById('div1');
  div1.innerText="123"
  div1.innerHTML='<strong>123</strong>'
</script>
</body>
</html>